Hello Gabriel,
superb Camera you have - I´ve got some Hasselblads too.
I don´t have any experience with Ixpress backs - but my
brother has a Phase One P25 for Hasselblad - its not square,
a 4:3 ratio (I´m not that sure) but the Hasselblad one would
be about the same 6x4.5 cm´s. Quality is AWESOME!!! Better
than film!
Btw - there may be some "bargains" buying a Hasselblad digital
back - you´ll get a new 500-camera for free (at least here in Germany) -
I think a 503CW or so.
hope I could help.
best regards
Andreas